Install

Test Doubles

A Test Double is an object that stands in for a real object in a test.
RSpec creates test doubles that support method stubs and message
expectations.

book = double("book")

Method Stubs

A method stub is an implementation that returns a pre-determined value. Method
stubs can be declared on test doubles or real objects using the same syntax.
rspec-mocks supports 3 forms for declaring method stubs:

You can also use this shortcut, which creates a test double and declares a
method stub in one statement:

book = double("book", :title => "The RSpec Book")

The first argment is a name, which is used for documentation and appears in
failure messages. If you don't care about the name, you can leave it out,
making the combined instantiation/stub declaration very terse:

double(:foo => 'bar')

This is particularly nice when providing a list of test doubles to a method
that iterates through them:

Message Expectations

A message expectation is an expectation that the test double will receive a
message some time before the example ends. If the message is received, the
expectation is satisfied. If not, the example fails.

Nomenclature

Mock Objects and Test Stubs

The names Mock Object and Test Stub suggest specialized Test Doubles. i.e.
a Test Stub is a Test Double that only supports method stubs, and a Mock
Object is a Test Double that supports message expectations and method
stubs.

There is a lot of overlapping nomenclature here, and there are many
variations of these patterns (fakes, spies, etc). Keep in mind that most of
the time we're talking about method-level concepts that are variations of
method stubs and message expectations, and we're applying to them to one
generic kind of object: a Test Double.

Test-Specific Extension

a.k.a. Partial Stub/Mock, a Test-Specific Extension is an extension of a
real object in a system that is instrumented with test-double like
behaviour in the context of a test. This technique is very common in Ruby
because we often see class objects acting as global namespaces for methods.
For example, in Rails:

person = double("person")
Person.stub(:find) { person }

In this case we're instrumenting Person to return the person object we've
defined whenever it receives the find message. We can do this with any
object in a system because rspec-mocks adds the stub and should_receive
methods to every object. When we use either, RSpec replaces the method
we're stubbing or mocking with it's own test-double-like method. At the
end of the example, RSpec verifies any message expectations, and then
restores the original methods.

Expecting Arguments

Argument Matchers

Arguments that are passed to with are compared with actual arguments
received using ==. In cases in which you want to specify things about the
arguments rather than the arguments themselves, you can use any of the
matchers that ship with rspec-expectations. They don't all make syntactic
sense (they were primarily designed for use with RSpec::Expectations), but
you are free to create your own custom RSpec::Matchers.

rspec-mocks also adds some keyword Symbols that you can use to
specify certain kinds of arguments:

double.should_receive(:msg).with(no_args())
double.should_receive(:msg).with(any_args())
double.should_receive(:msg).with(1, kind_of(Numeric), "b") #2nd argument can any kind of Numeric
double.should_receive(:msg).with(1, boolean(), "b") #2nd argument can true or false
double.should_receive(:msg).with(1, /abc/, "b") #2nd argument can be any String matching the submitted Regexp
double.should_receive(:msg).with(1, anything(), "b") #2nd argument can be anything at all
double.should_receive(:msg).with(1, ducktype(:abs, :div), "b")
#2nd argument can be object that responds to #abs and #div

Ordering

double.should_receive(:msg).ordered
double.should_receive(:other_msg).ordered
#This will fail if the messages are received out of order

Setting Reponses

Whether you are setting a message expectation or a method stub, you can
tell the object precisely how to respond. The most generic way is to pass
a block to stub or should_receive:

double.should_receive(:msg) { value }

When the double receives the msg message, it evaluates the block and returns
the result.

double.should_receive(:msg).and_return(value)
double.should_receive(:msg).exactly(3).times.and_return(value1, value2, value3)
# returns value1 the first time, value2 the second, etc
double.should_receive(:msg).and_raise(error)
#error can be an instantiated object or a class
#if it is a class, it must be instantiable with no args
double.should_receive(:msg).and_throw(:msg)
double.should_receive(:msg).and_yield(values,to,yield)
double.should_receive(:msg).and_yield(values,to,yield).and_yield(some,other,values,this,time)
# for methods that yield to a block multiple times

Arbitrary Handling

Once in a while you'll find that the available expectations don't solve the
particular problem you are trying to solve. Imagine that you expect the message
to come with an Array argument that has a specific length, but you don't care
what is in it. You could do this:

double.should_receive(:msg) do |arg|
arg.size.should eq(7)
end

Combining Expectation Details

Combining the message name with specific arguments, receive counts and responses
you can get quite a bit of detail in your expectations: